It is desirable to be able to power a optoelectronic circuit comprising light-emitting diodes with an AC voltage, particularly a sinusoidal voltage, for example, the mains voltage.
FIG. 1 shows an example of an optoelectronic circuit 10 comprising input terminals IN1 and IN2 having an AC voltage VIN applied between them. Optoelectronic circuit 10 further comprises a rectifying circuit 12 comprising a diode bridge 14, receiving voltage VIN and supplying a rectified voltage VALIM which powers light-emitting diodes 16, for example, series -assembled with a resistor 15. The current flowing through light-emitting diodes 16 is called IALIM.
FIG. 2 is a timing diagram of power supply voltage VALIM and of power supply current IALIM for an example where AC voltage VIN corresponds to a sinusoidal voltage. When voltage VALIM is greater than the sum of the threshold voltages of light-emitting diodes 16, light-emitting diodes 16 become conductive. Power supply current IALIM then follows power supply voltage VALIM. There is therefore an alternation of phases OFF without light emission and of light -emission phases ON.
A disadvantage is that as long as voltage VALIM is smaller than the sum of the threshold voltages of light-emitting diodes 16, no light is emitted by optoelectronic circuit 10. An observer may perceive this lack of light emission when the duration of each phase OFF with no light emission between two light-emission phases ON is too long. A possibility, to increase the duration of each phase ON, is to decrease the number of light-emitting diodes 16. A disadvantage then is that the proportion of electric power lost in the resistor is significant.
Publication US 2014/0252968 describes an optoelectronic circuit where the number of light-emitting diodes receiving power supply voltage VALIM progressively increases during a rising phase of the power supply voltage and progressively decreases during a falling phase of the power supply voltage. This is achieved by a switching device capable of short-circuiting a variable number of groups of light-emitting diodes according to the variation of voltage VALIM. This enables to decrease the duration of each phase with no light emission.
A disadvantage of the optoelectronic circuit described in publication US 2014/0252968 is that it requires using a difference amplifier for each group of light-emitting diodes. The circuit may thus have a high manufacturing cost. Another disadvantage is that the electric power consumption of the optoelectronic circuit may be significant. Another disadvantage is the complexity of the optoelectronic circuit, which may cause reliability issues.
Publication US-A-2013/0200802 describes an optoelectronic circuit comprising a plurality of series-assembled diodes and a switching device capable of short-circuiting a greater or lesser number of light emitting diodes according to the variation of the power supply voltage. The switching circuit comprises a differential amplifier.